A typical semiconductor wafer processing system has a process chamber for processing semiconductor wafers and wafer handling modules for moving the wafers in and out of the process chamber. Process chambers are available for chemical vapor deposition, physical vapor deposition, etching, electro-plating/electro-fill, plasma, thermal and radiation treatments, and other semiconductor device fabrication processes. For example, a chemical vapor deposition chamber may be used to deposit a film of dielectric material on a wafer.
Wafer handling modules employ manipulators, such as robots, for transporting wafers between two locations of a wafer processing system. A wafer placed in a loading station is picked-up by a robot and typically goes through a load lock before reaching the process chamber where the wafer is processed.
A load lock is often used to change the pressure from atmospheric at the loading station to the pressure required in process chambers (generally a reduced pressure). An atmospheric wafer handler removes a wafer from the load station and places it into the load lock through a port, valve, or door. After the arm of the wafer handler (usually called an “end effector”) exits, the load lock is isolated by closing the port, valve, or door. The small volume of the single-wafer load lock allows for fast pump down and vent cycles. The load lock is usually pumped down to the pressure of the next module, usually a process chamber or a transfer chamber. Then another port, valve, or door opens to equilibrate the pressure between the next module and the load lock, and the wafer passes through to the next module. Therefore, a wafer may transfer from atmospheric conditions to very low pressure conditions in a few seconds, while avoiding having to vent and pump down a larger volume process chamber or transfer chamber.
Load locks play a critical role in preventing contamination on the wafer. When the wafers are loaded to and unloaded from processing chambers during operation, residual particles are stirred and circulated as the processing chamber is pumped and/or vented to establish operating pressures or to restore loading pressures. Particles that can contaminate a wafer may be generated by the chemical or physical processes in the process chamber, scraping of wafer handling components, or chemical incompatibilities of the different materials. Wafers stay in the load lock until the processing chamber pressure is established. Because the load lock pressure and the pressure of the next module are controlled to be the same, when the wafer transfers, a minimum amount of pressure equalization takes place, so that residual particles are not stirred and contaminate the wafer. Additionally, load locks are not equipped with in situ cleaning capability. Technicians must normally perform any cleaning laboriously by dismantling the parts and cleaning them by hand. Because of the need to avoid contamination and difficulty to clean, no particle generating activity is normally performed in the load lock.
A load lock may also be equipped with a wafer heating or cooling unit and sometimes both. By providing a cooling unit within the load lock, system throughput can be increased by eliminating the need to move a newly processed wafer to a separate cooling station before moving the wafer to the load lock. System throughput for applications where the wafer needs to be heated before entering the process chamber can also be increased by providing a heating unit within the load lock. This eliminates the need for an intermediate pre-heating operation and station.
Load lock wafer heating has been accomplished with a heated pedestal. The heat is transmitted by gas conduction and radiation through a small gap between the wafer and the pedestal. But this wafer heating is inefficient because the pedestal has to be maintained at a much higher temperature than the desired wafer temperature. Maintaining a high pedestal temperature limits the types of materials that may be used and the life of the pedestal. Further, having such a high temperature object in close proximity to the wafer interferes with wafer temperature measurement for temperature control.
This technique of heating a wafer in a load lock is also slow. The heating time required to reach a steady-state temperature may be longer than the processing time in a process station, making the load lock pre-heat the throughput limiting process step, reducing tool throughput for some applications. An alternative is to remove a wafer before a steady-state temperature is reached, but because wafer temperature cannot be accurately measured, the downstream semiconductor process may be affected adversely if the wafer is too hot or too cold or if the temperature is not uniform across the wafer.
Improvements to this process and associated apparatus would be desirable.